In integrated circuit applications, more and more functions are integrated into products. For example, different functional elements such as 3G video elements, WiFi elements, Bluetooth elements, and audio/video elements may need to be integrated together to form an application. A commonly known application for these devices is the mobile application, in which mobile devices such as cell phones are formed.
High-frequency circuits, which include Radio Frequency (RF) passive devices, are widely used in the mobile applications. The RF passive devices may include capacitors, inductors, transformers, or the like. Due to the high frequency, various design issues were commonly observed. A common problem faced by designers is the signal loss in the substrates that are underlying the high-frequency circuits, which signal loss is partially caused by the parasitic capacitance between the high-frequency circuits and the underlying substrates. Typically, with the increase in the frequency of the signals, the signal loss becomes more severe. This significantly limits the design of high-frequency circuits.
Currently, there are a couple of solutions for reducing the substrate loss. For example, Silicon-On-Insulator (SOI) substrates were used by various groups of people to form the high-frequency circuits. Although the substrate loss may be reduced using this solution, the SOI substrates are typically expensive. In addition, the SOI substrates suffer from third harmonic issues, and hence the circuits formed thereon, for example, Complementary Metal-Oxide-Semiconductor (CMOS) devices, are very difficult to be integrated with the RF passive devices.
In addition, with the high frequency of the signals carried by the high-frequency circuits, the control circuits for operating the RF passive devices need to handle the quick change in the signals, and the control circuits need to have response time short enough to adapt to the change in the high-frequency signals.